Theory that the early farmers’ need for expanded trade routes caused humans to finally mingle genetically is included in this fascinating article that covers some new findings on our early farmers:

The new research also shows that even after agriculture was established across the Fertile Crescent, people remained genetically isolated for thousands of years.

“If they were talking to each other, they weren’t intermarrying,” said Garrett Hellenthal, a geneticist at University College London who collaborated with the Gutenberg University researchers.

But the DNA research also shows that this long period of isolation came to a sudden and spectacular end.
About 8,000 years ago, the barriers between peoples in the Fertile Crescent fell away, and genes began to flow across the entire region. The Near East became one homogeneous mix of people.

Why? Dr. Reich speculated that growing populations of farmers began linking to one another via trade networks. People moved along those routes and began to intermarry and have children together. Genes did not just flow across the Fertile Crescent — they also rippled outward. The scientists have detected DNA from the first farmers in living people on three continents.

Source: How the First Farmers Changed History – The New York Times